1. Field of the Invention:
The present invention relates to an optical pickup apparatus which reads information with laser beam from an information recording medium where information is recorded in the tracks formed thereon.
2. Description of the Prior Art:
FIG. 1 shows the outline of an existing optical pickup of the 3-beam system. In FIG. 1, reference numeral 1 denotes a disk formed as an information recording medium with information recorded in the tracks formed thereon. Reference numeral 2 is an optical pickup which reads information recorded on this disk. This optical pickup 2 is composed of an optical system and a photodetector 6 which detects a light flux (pencil of rays) reflected from an information track of the disk and guided by the optical system. The optical system includes a laser source 3, a diffraction grating (not shown), a beam splitter 4 and an objective lens 5 to irradiate the disk with the light flux radiated from the laser source 3 and condense the light flux on the information tracks. Moreover, although not shown, a drive mechanism which focuses the light flux onto an information track and causes the light flux to follow accurately the information track is also provided.
The photodetector 6, as shown in FIG. 2, is composed of at least four center photodetecting (receiving) sections A-D which are symmetrically arranged with the axes crossing each other almost orthogonally and two sub-photodetecting (receiving) sections E and F arranged to be opposed to each other on both sides of such center photodetecting sections. The outputs of these photodetecting sections are amplified by amplifiers 9-11 and the details thereof are shown in FIG. 2.
In FIG. 2, the preamplifiers 7 and 8 are respectively connected to the center photodetecting section pairs A, C and B, D which are respectively arranged in the diagonal direction among the center detecting sections A-D of the photodetector 6 and thereby photoelectric currents generated by these photodetecting sections are converted into voltages and these voltages are added. A preamplifier 9 is connected to two sub-photodetecting sections E and F arranged on both sides of photodetector 6 and outputs a tracking error signal by converting photoelectric currents generated by the sub-photodetecting sections into corresponding voltages and executing subtraction between such voltages. The outputs of preamplifiers 7 and 8 are added in an HF amplifier 10 to provide the information which is recorded on the disk 1. The outputs of preamplifiers 7 and 8 are also subtracted in an amplifier 11 to provide a focus error signal. These amplifiers are respectively formed on a circuit board 12 isolated from the pickup 2 and flexible shield wires 13 connect the circuit board 12 and the output terminals of pickup 2.
The output of each photodetecting section of the photodetector 6 is also connected, as shown in FIG. 3, to a preamplifier array 14 and the outputs of this array are used for adjusting the position of the photodetector 6 built in the pickup 2.
With reference to FIG. 1, FIG. 2, FIG. 3 and FIG. 4 (described later), the operation of a conventional optical pickup is explained hereunder.
FIG. 4 shows patterns of reflected light flux irradiating the photodetector 6. The light flux irradiating the photodetector 6 is composed of three beams, namely the main beam 15 which is intended to irradiate the center photodetecting sections A-D to provide information signals and focus error signals and two sub-beams 16a and 16b which irradiate the sub-photodetecting sections E and F to provide tracking error signals.
Before positional adjustment of photodetector 6, as shown in FIG. 4(a), the photodetector 6 is deviated to a considerable extent from the light flux. Therefore, a position detection of sub-beams is carried out first utilizing outputs of the sub-photodetecting sections E and F. As shown in FIG. 4(a), when the sub-photodetecting section E is irradiated with the sub-beam 16b, an output V.sub.Ea can be obtained from the preamplifier array 14 (shown in FIG. 3) corresponding to the sub-photodetecting section E. Here, V.sub.Ea means an output V.sub.E (FIG. 3) in FIG. 4(a). In this case, the amount of light of a sub-beam is set to be smaller than that of a main beam and therefore which beam irradiates the sub-photodetecting section E can be detected by checking the value of output V.sub.E.
Next, when said drive mechanism (not shown) causes the photodetector 6 to move to the position of FIG. 4(b) in the direction y in accordance with an output of the photodetector 6, the outputs V.sub.E and (V.sub.A +V.sub.B +V.sub.C +V.sub.D) of FIG. 3 yield the relation, V.sub.Eb &gt;(V.sub.A +V.sub.B +V.sub.C +V.sub.D).sub.b .apprxeq.V.sub.Ea because of the pattern of reflected light flux.
When the photodetector 6 is further moved to the position of FIG. 4(c) in the direction y, the outputs V.sub.E, (V.sub.A +V.sub.B +V.sub.C +V.sub.D) and V.sub.F yield the relation, (V.sub.A +V.sub.B +V.sub.C +V.sub.D).sub.c &gt;V.sub.Ec .apprxeq.V.sub.Fc because of the pattern of reflected light flux.
The coarse positional adjustment of the photodetector is completed by such manipulations. Next, the fine adjustment in the direction y can be undertaken by making equal the output voltages (V.sub.A +V.sub.B) and (V.sub.C +V.sub.D) of the photodetector and that in the direction x can also be accomplished by making equal the output voltages (V.sub.A +V.sub.D) and (V.sub.B +V.sub.C). As a result, as shown in FIG. 4(d), the photodetector 6 can be set to an appropriate position for the light flux reflected from the disk.
Upon completion of the positional adjustment of the photodetector 6 as explained above, the preamplifier array 14 for positional adjustment is separated and the preamplifiers for reading information signals stored on the disk, focus error signals and tracking error signals is in turn externally connected, as shown in FIG. 2, to the external output terminals of the pickup 2.
As explained above, the light flux emitted from the laser source 3 focuses on the disk 1 through the optical system and the reflected light flux from the disk is again guided by the optical system to the photodetector 6. When the reflected light enters the photodetector 6, it generates a photoelectric current between ten and twenty mircroamperes in accordance with the intensity of reflected light and reads the information recorded on the disk 1 based on such photoelectric current, automatically adjusting the focus and tracking of pickup 2 for the disk 1.
In other words, the light flux reflected from the disk 1 is changed in intensity to enter the center photodetecting sections A-D in response to the presence or absence of pits and accordingly the photoelectric currents generated by the center photodetecting sections also change. The preamplifiers 7 and 8 add such photoelectric currents and moreover the HF amplifier 10 adds the outputs of the preamplifiers to provide an output HF=(A+C)+(B+D) from which recorded information can be extracted.
If the light flux for the disk 1 is not focused correctly, irradiation patterns of reflected light flux incident on the center photodetecting sections A-D change and such change appears as a change of output in the photoelectric currents. Such out-of-focus can be corrected by adjusting the focus adjusting mechanism (not shown) disposed within the pickup 2 in accordance with an output of FE preamplifier 11 in the form of FE=(A+C)-(B+D).
In the case of a so-called optical pickup of the 3-beam system which irradiates the disk 1 with three divided light fluxes, one of the two light fluxes disposed to the front and rear of the main beam deviate from the target track, and the reflected light flux deviates from any one of the sub-photodetecting sections E, F on either side and does not enter the sub-photodetecting section correctly. In such a case, the TE preamplifier 9 outputs a signal in order to drive the tracking adjusting mechanism (not shown).
The pickup 2 thus follows the information tracks with rotation of the disk 1 and sequentially reads recorded information through movement in the radial direction.
The photodetector 6, as explained heretofore, is composed of at least four center photodetecting sections 6a-6d which are symmetrically arranged with the axes orthogonal to each other and two sub-photodetecting sections 6e, 6f arranged to be opposed to each other on both sides of said center photodetecting sections (FIG. 5). As shown in FIG. 5, the outputs of photodetector 6 are generally supplied to a current-voltage converting circuit 17 composed of CMOS inverters which are respectively connected to each other and correspond to the photodetecting sections 6a-6f to provide a voltage output converted from the current of the photodetecting sections to an amplifier in the succeeding stage.
FIG. 6 shows one form of the structure of the photodetector 6. As shown in FIG. 6, the photodetector 6 includes the photodetecting sections 6a-6f formed by high concentration p-type diffused layers (hereinafter referred to as p.sup.+ diffused layers) on a low concentration n-type impurity doped substrate 18 (hereinafter referred to as n.sup.- substrate).
Meanwhile, FIG. 7 shows one form of the structure of a CMOS inverter forming the current-voltage conversion circuit 17 and coupled to the photodetector 6. In FIG. 7, the reference numeral 19 denotes n.sup.- substrate; 20 is p.sup.+ diffused layer; 21, 22 are oxide films; 23 is p.sup.+ diffused layer; 24 is p.sup.- diffused layer; 25, 26 are n.sup.- diffused layers; 27, 28 are conductive electrodes.
The laser beam emitted from the laser source 3 is divided by the diffraction grating (not shown) into one main beam and two sub-beams and these beams irradiate the tracks on the disk 1 through the objective lens 5. On the other hand, the reflected light from the disk 1 enters the objective lens 5, is divided by the beam splitter 4 and enters the photodetector 6. Namely, as shown in FIG. 5, the main beam irradiates the center photodetecting sections 6a-6d of the photodetector 6 and the sub-beams respectively irradiate the sub-photodetecting sections 6e and 6f which oppose each other.
Therefore, each of the photodetecting sections 6a-6f of the photodetector 6 supply photoelectric currents in accordance with the amount of light to the voltage converting circuit 17 from the external terminals and these photoelectric currents are converted therein into voltage signals. An electrical circuit of such voltage conversion circuit is shown in FIG. 8. At the detecting part of photodetector 6, a bias power source (V.sub.cc) is connected through a resistor 29 and a capacitor 30 in order to prevent interference between output signals of photodetecting sections and a photoelectric current I is generated when the photodetecting sections are irradiated with light flux.